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Problem 412

Combinatorics Difficulty 2.7 Multiple choice CEMC Gauss (Grade 8) · Canada · 2013

Serena colours the hexagons on the tiling shown.

If two hexagons share a side, she colours them with different colours. What is the least number of colours that she can use to colour all of the hexagons?

Pick one

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Official solution

Using the special six-sided die, the probability of rolling a number that is a multiple of three is 12\frac{1}{2}.

Since 12\frac{1}{2} of 6 is 3, then exactly 3 numbers on the die must be multiples of 3.

Since the probability of rolling an even number is 13\frac{1}{3} and 13\frac{1}{3} of 6 is 2, then exactly 2 numbers on the die must be even.

The die in (A) has only 2 numbers that are multiples of 3 (3 and 6), and thus may be eliminated.

The die in (C) has 4 numbers that are even (2,4,6,62,4,6,6), and thus may be eliminated.

The die in (D) has 3 numbers that are even (2,4,62,4,6), and thus may be eliminated.

The die in (E) has 4 numbers that are multiples of 3 (3,3,3,63,3,3,6), and thus may be eliminated.
The die in (B) has exactly 3 numbers that are multiples of 3 (3,3,63,3,6), and exactly 2 even numbers (2 and 6), and is therefore the correct answer.

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.